1. Field of the Invention
This invention relates to a heat exchanger and more particularly, but not exclusively, to a shell and tube heat exchanger configured to provide for a uniform velocity of fluid flow along a helical path and a maximized heat transfer.
2. Summary of the Invention
A constant battle for maximizing production by heat-exchanging and/or heat-generating assemblies primarily target to achieve the following:
Higher heat transfer efficiency;
Lower pressure drop;
Increased performance;
Effective protection against vibration; and
Reduced installation and maintenance costs.
Whether it is the offshore, refinery, power, petrochemical or paper and food industries, heat exchangers are often the core of the above-enumerated objectives. Numerous configurations of the heat exchanger are known and used for a variety of applications. One of the widely used configurations of the heat exchanger-a shell and tube heat exchanger of FIG. 1-comprises a cylindrical shell 10 housing a bundle of parallel pipes 12, which extend between two end plates 14 so that a first fluid 16 can pass through the pipes 12. Meanwhile, a second fluid 18 flows in and through the space between the two end plates so as to come into contact with the pipes. To provide an improved heat exchange between the two fluids, the flow of the second fluid 18 is defined by intermediate baffles 20 forming respective passages, which are arranged so that the second fluid flow changes its direction in passing from one passage to the next. The baffles 20, configured as annular rings and discs, are installed perpendicular to a longitudinal axis 22 of the shell 10 to provide a zigzag flow 24 of the second fluid 18.
Disadvantageously, the second fluid has to sharply change the direction of its flow several times along the length of the shell. This causes a reduction in the dynamic pressure of the second fluid and non-uniform flow velocity thereof, which, in combination, adversely affect the performance of the heat exchanger.
A scientific community has long been aware that a perpendicular position of baffles relative to the longitudinal axis of the shell is largely responsible for a relatively inefficient heat transfer rate/pressure drop ratio. Adjacent baffles extending parallel to one another and at a right angle with respect to the longitudinal axis of the shell define a cross flow path characterized by numerous sharp turns between adjacent channels. The efficiency of heat transfer can be improved by reducing the spacing or window between the baffles. However, decreasing the window results in high flow velocity along the outer edges of the baffles, which are juxtaposed with the shell, and low flow velocity closer to the center of the shell. The non-uniformity of flow distribution within each segment defined between the adjacent baffles causes numerous eddies, stagnation regions as well expansion/contraction of pipe stretches, which decrease convective heat transfer rates. A further factor contributing to a decreased heat transfer rate is attributed to the fact that the pipes traversed by the first fluid have to be positioned at a certain radial distance from the shell. Accordingly, the cross flow around the peripherally located pipes is faster than around centrally mounted pipes.
Thus, conventional baffle arrangement as described above results in flow bypass through baffle-to-shell and pipe-to-baffles clearances. Bypass flow reduces the cross-flow heat transfer while the flow maldistribution caused by significant velocity variations increases back-flow and eddies in the dead zones, and consequently higher rates of fouling on the shellside. Such flow maldistribution leads to the high temperatures and corrosion of the peripheral pipes causing their rapid deterioration and, as a consequence, the reduced role in the heat exchange process. Since the heat exchanger design is based on the uniform contribution of each pipe of the entire bundle to the heat exchange process, those pipes that have been damaged cannot meet this requirement and should be replaced. Costs associated with such replacement are high making the maintenance of the heat exchanger cost prohibitive.
Furthermore, conventional arrangement may cause high flow-induced vibration losses since long pipes reaching often 24-feet long are supported by a succession of baffles which, in order to solve the problem associated with the non-uniform velocity, are spaced apart at a substantial distance. As a result of high thermal gradient and non-uniform cross flow vibration hazards are significant.
Thus, it is desirable to configure a baffle assembly that can attain the following objectives:
Uniformity of cross-flow through a shell leading to an improved convection heat exchange rate;
Stability and correctness of actual positioning of multiple baffles relative to multiple pipes supported by a baffle assembly or cage; and
Facilitation of installment of a baffle assembly.
These objectives have been achieved by replacing conventional segmental plate baffles with a succession of spaced apart quadrant-shaped baffles each positioned at an angle to a longitudinal axis of a shell to create a pseudo helical flow path on the shellside. One of the advantages of the inventive structure is that the angularly positioned baffles act as guide vanes for the cross flow, which has substantially uniform velocity along the opposite sides of each baffle avoiding thus back flow and eddies.
Thus, instead of squeezing the cross flow as done in the above-discussed conventional design, a succession of inclined baffles directs the second fluid along a helical, more natural flow path providing for a substantially uniform flow rate and minimization of leakages. Since the flow velocity is substantially uniform on both sides of each baffle, a pressure gradient across the latter is insignificant. Hence, there are no undesirable leakages across or through the baffles, and the flow, as theoretically designed, occurs mainly along the surface of the baffles, which face the inner wall of the shell and form the peaks of the helical path. Thus, while the second fluid can traverse the entire length of the shell faster or slower depending on the angle of the baffles relative to the normal to the longitudinal axis of the shell, the flow velocity remains constant.
Furthermore, since flow energy consumed in expansion and contraction of flow conveying elements is minimal, the pressure losses are merely a fraction of the losses observed in the conventionally baffled heat exchangers. Thus, the helical baffle geometry offers much higher conversion of available pressure drop to heat transfer.
In accordance with one aspect of the invention, helical baffle quadrants reflect the segments of elliptical plates. Configuration of the elliptically shaped outer surfaces juxtaposed with the inner wall of the shell provides for tight clearances therebetween and, as a consequence, minimizes leakages when the helically baffled tube bundle is inserted into the shell.
To ensure the desired positioning of multiple baffles relative to one another and to a bundle of pipes subsequently mounted through these baffles, the invention provides for variously configured reinforcing elements interconnecting a succession of baffles. In accordance with one embodiment, separate longitudinal seal strips are tack welded to the baffle edges of adjacent baffles. Alternatively, spacer strips can bridge tie rods, which are configured to secure the spaced-apart baffles. Finally, the opposite radial flanks of each baffle may have an angularly extending flange provided with fully formed holes that are traversed by those pipes that would otherwise be secured in open semi holes formed along opposing edges of the adjacent baffles.
Still a further aspect of the invention provides for a helical baffle arrangement including two strings of baffles, which form a double helix pattern. Such a structure is particularly advantageous for reinforcing longs spans of pipes, without, however, affecting the uniform velocity of the flow.
The inventive structure is equally advantageous for existing plants as well as for grassroots applications. For the former, the advantage of the inventive structure is that it helps to increase the capacity while lowering maintenance costs. Indeed, the percentage of pipes needed to be replaced due to the corrosion and mechanical failure is substantially reduced as a result of elimination of eddies or back mixing. For the grassroots applications, the inventive structure helps to reduce plot space, energy costs and investment.
It is therefore an object of the invention to provide an improved baffle arrangement in a shell and pipe heat exchanger configured to minimize the non-uniformity of the cross flow velocity and to maximize the heat exchange rate;
Still a further object of the invention is to provide a quadrant baffle plate shaped to minimize clearances between the baffle arrangement the inner side of the shell;
Yet another object of the invention is to provide a succession of quadrant baffles with reinforcing arrangements configured to facilitate insertion and ensure the desired position of the pipes in the quadrant baffles;
A further object of the invention is to provide a double helix arrangement of the quadrant baffles configured to enhance bundle integrity against flow-induced vibrations; and
Still a further object of the invention is to configure the quadrant baffles so that the double helix arrangement installation would be labor effective.